1. Field of the Invention
The present invention relates to a power supply having a power limit of less than 100 VA, yet still capable of supplying for a short duration an instantaneous load demand that is larger than its power limit.
2. Description of Related Art
Power supplies are available as off-the-shelf electronic components meeting the “Limited Power Source” requirement of applicable UL specifications, more particularly, of UL 1950, aimed at eliminating the possibility of the power supply causing hazardous fires when overloaded or malfunctioning. “Limited power” is generally defined as power not exceeding 100 VA.
To meet such specifications, “fuses” have been employed in the prior art that blow open when the load element exceeds 100 VA. This technology is however undesirable, because a fuse needs to be replaced every time a fault occurs, and because the fuse reaction time to an overload condition is relatively long, possibly causing unwanted damage to the equipment to which the power supply is supplying power.
Another type of power supply in the prior art that meets the “Limited Power Source” requirement uses an internal electronic current limiting the total power output so that it never exceed 100 VA. This technology provides operating disadvantages, because some types of electronic equipment demand a high momentary current of a relative short duration, with a peak power demand as high as 400-500 VA, while the average power demand in a longer time span is always less than 100 VA. This causes this type of power supply, rated less than 100 VA, useless for such applications.
Further, while UL 1950 requires a reaction time of 30 seconds, a revision of UL 1950 presently under review will decrease the reaction time to five seconds.